In prior art, a bumper reinforcement as a core member of a bumper of an automobile is detachably mounted on an end of a side member included in a part of a body shell, for example, by interposing a bonded channel-shaped bracket using appropriate means such as fasteners. Meanwhile, in recent years, the bracket is substituted with a crash box in order to improve safety of the chassis and lower a repair cost by avoiding a serious damage of the chassis in a light collision. The crash box has a tubular body buckled earlier than other member in an axial direction by an impact load exerted in its axial direction (herein, meaning a “longitudinal direction of the crash box”) and plastically deformed in a (accordion-like) bellows shape to absorb impact energy.
The impact absorbing performance required in the crash box can be determined specifically based on the following factors.                (a) If the impact load is exerted in the axial direction, the crash box is repeatedly and stably buckled in the axial direction to reliably generate plastic deformation in a bellows shape.        (b) An average load is high when the crash box is crushed.        (c) A maximum reactive force generated in the crushing is suppressed so as not to break down a side member that supports the crash box.        
For example, Patent Literatures 1 to 5 disclose various materials and shapes for improving the impact absorbing performance of the crash box. However, even in any one of the prior arts described above, it is not easy to repeatedly and stably buckle the body in its axial direction due to the exerted impact load and plastically deform it in a bellows shape without increasing a weight by adding a partitioning wall or increasing a sheet thickness.
The applicant proposed a crash box 1 in Patent Literature 6. Referring to the cross section of FIG. 13, at least a part of the cross-sectional shape in an axial direction is a closed cross section having a generally polygonal shape, and there is no flange in the outer side of the closed cross section. Trench portions 3 and 4 extending in an axial direction of the tubular main body 2 are provided in a part of the area of a pair of sides D-I and L-A of a basic cross section defined as a polygon A-B-C-D-I-J-K-L-A having a maximum area out of polygons obtained by straightly linking a part of end points A to P of the approximate polygon, by forming an inwardly convex shape on the basic cross section A-B-C-D-I-J-K-L-A in a position other than the end points D, I, L, and A.
In the crash box 1, it is possible to obtain a high impact energy absorption amount by stably generating buckling and plastic deformation in a bellows shape against an impact load exerted in an axial direction without increasing a weight by adding a partitioning wall or increasing a sheet thickness, or generating bending in an axial direction.
In an actual automobile crash accident, an impact load caused by the crash is not always input in the axial direction of the crash box 1 continuously from the start to the end of plastic buckling deformation of the crash box 1 in a bellows shape. Instead, in many cases, the impact load is exerted obliquely with respect to the axial direction of the crash box 1. Although the crash box 1 stably and reliably generates plastic buckling deformation in a bellows shape across the entire area in the axial direction against an impact load exerted in the axial direction of the main body 2, an impact load exerted obliquely with respect to the axial direction easily generates strong bending deformation in the entire main body 2 in the middle of the plastic buckling deformation of a bellows shape due to a bending moment generated in the main body 2. Then, it is difficult to generate plastic buckling deformation in a bellows shape. Therefore, the impact energy absorption performance is degraded accordingly.
In this regard, the applicant disclosed a crash box 5 described below as illustrated in FIG. 14 in Patent Literature 7. That is, the crash box 5 includes a tubular main body 10 having a pair of corner portions 6 and 7 arranged oppositely and another pair of corner portions 8 and 9 arranged perpendicularly to a line L1 obtained by linking the corner portions 6 and 7. The tubular body 10 has a rectangular cross-sectional shape having no flange in the outer side. Here, an angle θ1 of the pair of corner portions 6 and 7 is set to 90° or larger and 150° or smaller, and an angle of the another pair of corner portions 8 and 9 is set to 30° or larger and 90° or smaller.
The cross-sectional shape of the crash box 5 has one or more inwardly convex trench portions 11 to 14 that extend in a longitudinal direction in a position other than the pair of corner portions 6 and 7 and the another pair of corner portions 8 and 9, is symmetrical with respect to a line passing through the pair of corner portions 6 and 7, and is provided in each of at least one out of two pairs of sides arranged symmetrically with respect to the line L1 passing through the pair of corner portions 6 and 7. In addition, each of the sides satisfies a relationship “5<(W−N×Wc)/(N+1)/t<50”, where “t” denotes a sheet thickness (mm), “W” denotes a length of the side (mm), “N” denotes the number of trench portions 11 to 14, and “Wc” denotes an average of opening widths of the N trench portions (mm).
The applicant disclosed a crash box 15 described below in Patent Literature 8 as illustrated in FIG. 15 in order to further improve the performance of the crash box 5. Specifically, the crash box 15 has a pair of corner portions 16 and 17 arranged oppositely and another pair of corner portions 18 and 19 arranged to intersect with a line obtained by linking the pair of corner portions 16 and 17 at an angle of 80° to 100°. In addition, the crash box 15 is formed from a metal tubular body 20 having a rectangular basic cross-sectional shape, so that an impact load is exerted from one end to the other end in an axial direction of the tubular body 20.
In addition, in the crash box 15, an angle of the pair of corner portions 16 and 17 is set to 90° or larger and 150° or smaller, and an angle of the another pair of corner portions 18 and 19 is set to 30° or larger and 90° or smaller. In addition, the crash box 15 has one or more inwardly convex trench portions 21 to 24 extending in a longitudinal direction in a position other than the pair of corner portions 16 and 17 and the another pair of corner portions 18 and 19. Furthermore, a cross-sectional peripheral length of the tubular body 20 in one end side is shorter than a cross-sectional peripheral length of the tubular body 20 in the other end side.
In the crash box 15, it is possible to continuously and stably generate plastic buckling deformation in a bellows shape even when an impact load caused by a crash is exerted obliquely as well as in a direction parallel to the axis direction. As a result, the crash box 15 has an excellent impact absorption characteristic (herein, referred to as “robustness”), that is, a large absorption amount of impact absorption energy even for an oblique crash.
In the crash box 15, crushing of the tubular body 20 reliably progresses from the impact end side when an impact force is exerted to the tubular body 20. Even when an impact load caused by a crash is exerted obliquely from an axial direction of the tubular body 20, crushing of the tubular body 20 propagates from one end where the impact load is exerted to the other end in the axial direction of the tubular body 20, so that the tubular body 20 can reliably and effectively buckled and deformed in a bellows shape to absorb impact energy.